Method for balancing a flow of liquid steel into a casting mold and continuous casting system for liquid steel

ABSTRACT

This method for balancing a flow of liquid steel into a casting mold, in which the steel is introduced into the casting mold from a tundish through a protective nozzle which opens below the steel level into the casting mold, comprises the following steps: a) acquiring a set of characteristics of the flow in the casting mold, b) comparing the flow characteristics acquired in the previous step with a predefined model and determining the adjustment actions to take in order to balance the flow, and c) adjusting the flow.

The invention relates to an installation for the continuous casting of metals. More particularly, the invention relates to a method for balancing a flow of liquid steel in a casting mold. In another of its aspects, the invention relates to a system for the continuous casting of liquid steel.

An installation for the continuous casting of metals, for example an installation for the continuous casting of steel, generally comprises a casting mold into which a liquid metal is poured from a pouring base or a tundish so that it will solidify in a suitable shape. This may be a bottomless casting mold, in which case the metal cools to form a slab. In order to cool the liquid metal, walls of the casting mold adjoin, or are backed by, cooling devices, for example of the liquid-cooled type. The casting mold and the cooling devices are sized according to the rate of flow of the metal so that the slab, on leaving the casting mold, has a solidified external shell thick enough to trap the still-liquid metal that is at the heart of the slab.

The tundish is equipped with one, or even several, nozzles below the level of steel in the casting mold intended to protect the liquid metal as it flows toward the casting mold. In general, the nozzle is positioned symmetrically with respect to the casting mold so that the flow is as uniform as possible during the continuous casting operations. This is because an unbalanced flow in the casting mold may have negative consequences on the quality of the slab, such as the risks of breakout, heterogeneity in the cast steel, poor distribution of the lubricating powder, etc.

Nevertheless, certain incidents may disturb the balance of the flow of liquid steel from the tundish into the casting mold. For example, one of the openings of the nozzle may become eroded or plugged with alumina, steel may solidify in the nozzle, or debris may become lodged in the nozzle. All of these incidents have the effect of disturbing the symmetry of the flow and therefore potentially of impairing the quality of the slabs produced, or even of damaging the continuous casting installation. To date, there is no solution for detecting such situations, and even less so for remedying them.

It is an object of the invention to allow incidents disturbing the flow of the liquid steel to be detected, and the symmetry of the flow to be reestablished.

To this end, the invention provides a method for balancing a flow of liquid steel in a casting mold, wherein the steel is introduced into the casting mold from a tundish through a protective nozzle opening below the level of steel in the casting mold, comprising the following steps:

-   -   a) acquisition of a set of characteristics of the flow in the         casting mold,     -   b) comparison of the flow characteristics acquired in the         preceding step against a predetermined model and determination         of the adjustment actions to be taken in order to balance the         flow, and     -   c) adjustment of the flow.

Thus, it is possible to determine whether the flow is disturbed by measuring characteristics of the flow and comparing these measurements against a predefined model. The quality of the flow can therefore be evaluated almost instantaneously, and if a disturbance, namely a sufficiently significant discrepancy between the measured characteristics and the model, arises, it is possible to react by adjusting the flow in such a way as to reduce the disturbance. The quality of the slabs produced is thereby significantly improved.

Advantageously, steps a) to c) are repeated continuously during the casting operations.

The method can thus be implemented throughout the period of operation of the continuous casting installation.

Advantageously, the flow characteristics are obtained by an analysis of the thermal characteristics of the steel in the casting mold.

Because casting-mold temperature is easily measurable at a great many positions, this contributes to making the method easy to implement.

Advantageously, the casting mold is of the type consisting of an assembly of metal plates backed by cooling devices which are configured to allow the metal plates to be cooled by the circulation of a cooling fluid, comprising an optical fiber, comprising a plurality of Bragg filters, extending in a wall of at least one of said plates, the optical fiber extending in a direction not parallel to the pouring axis of the casting mold.

Advantageously, the method further comprises the following steps:

-   -   measurement of the temperature of at least one wall of the         casting mold by means of the optical fiber, and     -   adjustment of the flow.

The temperature is thus measured using the optical fiber, which is reliable and easy to install in the casting mold. In particular, it is possible to use a casting mold like the one described in Belgian patent application 2018/5193 or in the Belgian patent application filed simultaneously with the present application.

Advantageously, the flow adjustment is performed by effecting a relative movement between the nozzle and the casting mold.

As a preference, the relative movement between the nozzle and the casting mold is effected in a direction parallel to the longitudinal axis of the casting mold.

Advantageously, the nozzle is secured to the tundish and the relative movement between the nozzle and the casting mold is achieved by moving the tundish with respect to the casting mold. For example by effecting a small movement of the tundish car.

According to a variant of the invention, the relative movement between the nozzle and the casting mold is effected by angularly offsetting the nozzle about the longitudinal axis of the casting mold. It is also possible to combine the two movements (linear and angular).

As a variant, in the case where the tundish is provided with a device for replacing the casting nozzle or for regulating the flow of steel by restricting it by means of a plate moved perpendicularly to the direction of flow, it is sufficient to move such a device with respect to the casting mold.

The flow adjustment is thus achieved through an operation that is simple to implement.

The invention also provides a system for the continuous casting of liquid steel from a tundish to a continuous casting mold, comprising:

-   -   a tundish,     -   a casting mold of the type consisting of an assembly of metal         plates backed by cooling devices which are configured to allow         the metal plates to be cooled by the circulation of a cooling         fluid, comprising an optical fiber, comprising a plurality of         Bragg filters, extending in a wall of at least one of said         plates, the optical fiber extending in a direction not parallel         to the pouring axis of the casting mold,     -   a protective nozzle the lower end of which opens below the level         of the steel in the casting mold while the steel is being         poured, the nozzle being secured to the tundish,     -   an emitter-receiver designed to send light into the optical         fiber and to receive the light reflected and/or transmitted by         the optical fiber,     -   a processor designed to:         -   a) convert the data pertaining to the reflected and/or             transmitted light received by the emitter-receiver into             information pertaining to the flow in the casting mold,         -   b) compare this information against a predefined model,         -   c) determine the adjustment actions to be taken in order to             balance the flow,         -   d) emit a control signal,     -   adjustment means designed to receive the control signal and to         adjust the flow of the steel in the casting mold as a function         of the control signal.

Advantageously, the adjustment means comprise a tundish car.

The adjustment means are thus formed by simple means.

One embodiment of the invention which is given purely by way of nonlimiting example will now be set out with the support of the attached figures in which:

FIG. 1 is an overall view of an installation for the continuous casting of metals allowing implementation of a method for balancing a flow of liquid steel in a casting mold according to the invention,

FIGS. 2a and 2b are diagrams illustrating the operation of the installation of FIG. 1,

FIG. 3 is a view in cross section of the casting mold of the installation of FIG. 1,

FIG. 4 is a perspective view of a plate of the casting mold of FIG. 3,

FIG. 5 is a view in longitudinal section of an optical fiber contained within the wall of FIG. 4,

FIG. 6 is a diagram explaining the operation of the optical fiber of FIG. 5, and

FIG. 7 is a view on a larger scale of the installation of FIG. 1 illustrating the implementation of the method for balancing the flow of liquid steel in the casting mold.

FIG. 1 depicts an installation 2 for the continuous casting of metals. Its configuration is conventional which means that most of its constituent elements will be introduced only succinctly.

The installation 2 comprises ladles 4 containing liquid metal that needs to be cooled. Here the ladles 4 are two in number and are carried by a motorized arm 6. This motorized arm 6 is notably able to move the ladles 4 which are brought full into the casting zone by a transport system (for example a traveling train, not depicted) from a filling zone in which the molten metal can be tipped into them, for example a furnace or a converter (not depicted) before they are brought into the position illustrated in FIG. 1. After the ladle 4 has been emptied, the motorized arm 6 also allows the empty ladle to be positioned in a position in which the transport system will take charge of it once again and convey it to a preparation zone where it can be reconditioned before returning to the filling zone.

The installation 2 comprises a tundish or pouring basin 8 situated beneath the ladles 4. The latter have a bottom that can be opened to cause the liquid metal to pour out into the tundish 8.

The tundish 8 comprises an outflow orifice which may be stopped by a stopper rod 10 for controlling the flow of liquid metal. The outflow orifice of the tundish is extended by a protective nozzle 11 (also known as a submerged entry nozzle, SEN) protecting the poured out liquid metal. The nozzle 11 is secured to the tundish 8.

As is more clearly visible in FIG. 2a and on a larger scale in FIG. 2b , the nozzle 11 opens into an upper opening of a casting mold 12. In this instance it is a bottomless casting mold having a pouring axis that is vertical. The casting mold 12 will be described in greater detail later.

The installation 2 comprises cooling devices 14 positioned on an external surface of the casting mold 12. These are cooling devices of the liquid-cooled type. To this end they comprise ducts into which a refrigerating fluid, for example water, flows. The refrigerating fluid absorbs the heat of the liquid metal in the casting mold 12 in order to cool and solidify this metal. Here, the metal solidifies in the form of a slab having a solidified external shell 18 enclosing a liquid core 20.

The installation 2 comprises a roller guide 16 downstream of the casting mold 12. The guide 16 guides the slab, an external shell 18 of which has solidified, from the casting mold 12. As is visible in FIG. 2a , the slab solidifies progressively as it moves along the guide 16. In other words, the more it moves away from the casting mold 12, the more the solidified external shell 18 of the slab increases in volume and the more the liquid core 20 of the slab decreases in volume.

The casting mold 12 is depicted in greater detail in FIG. 3. In this instance, it has four plates 22 (the fourth not being visible because of the position of the plane of section). The plates 22 are made from copper or copper alloy, which materials exhibiting high thermal conductivity and therefore facilitating the changes of heat between the cooling devices 14 and the casting mold 12. The plates 22 are arranged in such a way that the casting mold 12 has a right cross section that is rectangular or square overall. However, provision could be made for the plates to be arranged in such a way that the casting mold has any other shape of right or non-right cross section. For example, a funnel-shaped upper section conventionally used for casting thin slabs.

In what follows, for the sake of conciseness, the invention will be described in greater detail on the basis of a casting-mold arrangement like the one described in Belgian patent application 2018/5193, namely with an optical fiber housed inside a duct formed in the wall of the casting mold. However, it must be appreciated that, according to another embodiment of the invention, the optical fiber may be housed in a groove formed in the surface of the casting mold and closed by a strip, as described in the Belgian patent application filed simultaneously with the present application.

One of the plates 22 of the casting mold 12 is depicted on a larger scale in FIG. 4, in which the pouring axis corresponds to the vertical direction. The plate 22 comprises in its wall at least one duct 24 extending in a direction not parallel to the pouring axis of the casting mold 12. More specifically, the duct 24 is at an angle of between 75° and 105° with respect to the pouring axis. In this instance, the duct 24 is perpendicular to the pouring axis. The ducts 24 are four in number here. A protective cap 26 is installed over the zone of the plate 22 at which the ducts 24 open in order to protect these.

An optical fiber 28 is housed in each of the ducts 24. With reference to FIGS. 5 and 6, each optical fiber 28 comprises an optical sheath 30 and a core 32 surrounded by the optical sheath 30. The optical fiber 28 comprises, in its core 32, a plurality of Bragg filters 34. The optical fiber 28 comprises at least ten Bragg filters 10 per meter, preferably at least twenty Bragg filters per meter, and as a preference at least thirty Bragg filters per meter, and more preferably still, at least forty Bragg filters per meter. By way of variant embodiment, provision could be made for the casting mold to contain just one single optical fiber. In what follows, the installation 2 will be considered to comprise just one optical fiber in order to make the description thereof easier.

The operation of the optical fiber 28 is illustrated in FIG. 6. Bragg filters 34 are filters able to reflect light over a range of wavelengths which is centered on a predetermined value, known as the reflected wavelength, that can be adjusted by the manufacturer of the filter. This predetermined value is also dependent notably on the temperature at which the filter finds itself, which means that, for each filter, it is possible to write:

λ_(reflected) =f(λ₀ ,T)

where λ_(reflected) is the wavelength effectively reflected by the filter, f is a known function, T is the temperature of the filter and λ₀ is the wavelength reflected by the filter at a predetermined temperature, for example at ambient temperature.

These two properties mean that the optical fiber 28 can be used as a temperature sensor. Initially, Bragg filters 34 having distinct and chosen reflected wavelength values λ₀, for example offset from one another by 5 nanometers, are installed in the optical filter 28. A beam of light 35 a exhibiting a polychromatic spectrum, for example white light, is then sent into the optical fiber 28 and then the wavelength peaks represented in the spectrum of the reflected beam 35 b are determined. For each peak, the measured value λ_(reflected) and the theoretical value of the wavelength reflected at ambient temperature λ₀ are compared, and the temperature T of the filter in question is calculated using the function f. Alternatively, it is also possible to carry out these steps on the basis of the troughs in the spectrum of the transmitted beam 35 c if the configuration of the duct 24 in which the optical fiber 28 is housed permits this.

Thus, installing the optical fiber 28 in one of the plates 22 of the casting mold 12 allows the temperature of this plate, and notably of its wall in contact with the poured metal, to be measured at predetermined positions and to monitor how this evolves over time. In order to obtain a sufficiently high number of measurement points, it is preferable to position at least one optical fiber 28 in two opposing plates 22, or even in each of the four plates 22 of the casting mold 12.

For the purposes of balancing the flow of the liquid steel in the casting mold 12, the installation 2 further comprises:

-   -   an emitter-receiver designed to send light into the optical         fiber 28 and to receive the light reflected and/or transmitted         by the optical fiber 28,         -   a processor designed to:         -   a) convert the data pertaining to the reflected and/or             transmitted light received by the emitter-receiver into             information pertaining to the flow in the casting mold,         -   b) compare this information against a predefined model,         -   c) determine the adjustment actions to be taken in order to             balance the flow,         -   d) emit a control signal to an adjustment system, and     -   an adjustment system designed to adjust the flow of the steel in         the casting mold 12 as a function of a control signal emitted by         the processor.

The operation of these elements will be described in what follows.

At any moment during the flow, measurements of a set of characteristics of the flow in the casting mold 12 are taken. In particular, the emitter-receiver sends light into the optical fiber 28 and the temperature of the wall of the casting mold 12 is measured using the light reflected and/or transmitted by the optical fiber 28. However, more generally, thermal characteristics of the steel present in the casting mold 12 are analyzed.

The processor is then used to compare the measurement of these characteristics against a predefined model. These may, for example, be concerned with measurements of these same characteristics taken previously under normal flow conditions, namely conditions in which the flow is not disturbed.

If the measurement does not deviate from the model by a predetermined amount, the comparison is interpreted as signifying that no disturbance of the flow is occurring. No flow adjustment measure therefore needs to be undertaken. These measurement and comparison steps are preferably repeated continuously throughout the pour.

If the opposite is true, the comparison is interpreted as signifying that at least one disturbance has occurred and that the flow therefore needs to be adjusted. Taking the comparison into consideration, the processor determines the adjustment actions to be taken in order to balance the flow and then emits a control signal to adjustment means that allow adjustment actions to be undertaken.

If the processor detects a measurement that deviates excessively from the model, provision may be made for an alarm signal to be emitted or even for the casting operations to be halted.

The adjustment actions may consist in moving the tundish 8 in a direction parallel to the longitudinal axis of the casting mold 12 using a tundish car 36 of the installation 2. Given that the nozzle 11 is secured to the tundish 8, this movement allows the nozzle 11 to be moved with respect to the casting mold 12. In doing this, symmetry in the flow of the liquid metal is reestablished.

The measurement and comparison steps are then performed again in order to determine whether the moving of the nozzle 11 has had the anticipated effect. Provision may be made for this movement to continue as long as the discrepancy between the measurement and the model remains greater than the predetermined amount. Once this amount becomes smaller than the predetermined amount, the tundish car is deactivated so that the movement of the nozzle 11 is halted. However, the measurement and comparison operations continue to be performed in order to detect any further incident that might arise.

The invention is not restricted to the embodiments described and other embodiments will be clearly apparent to those skilled in the art.

PARTS LIST

-   -   2: installation (for the continuous casting of metals)     -   4: ladle     -   6: motorized arm     -   8: tundish     -   10: stopper rod     -   11: protective nozzle     -   12: casting mold     -   14: cooling devices     -   16: guide     -   18: solidified outer shell     -   20: liquid core     -   22: plate     -   24: duct     -   26: protective cap     -   28: optical fiber     -   30: optical sheath     -   32: core     -   34: Bragg filter     -   35 a: polychromatic spectrum     -   35 b: spectrum of the reflected beam     -   36 c: spectrum of the transmitted beam     -   36: tundish car 

1. A method for balancing a flow of liquid steel in a casting mold (12) having a longitudinal axis, wherein the steel is introduced into the casting mold (12) from a tundish (8) through a protective nozzle (11) opening below the level of steel in the casting mold (12), comprising the following steps: a) acquisition of a set of characteristics of the flow in the casting mold (12), b) comparison of the flow characteristics acquired in the preceding step against a predetermined model and determination of the adjustment actions to be taken in order to balance the flow, and c) adjustment of the flow; wherein the flow adjustment is performed by effecting a relative movement between the nozzle (11) and the casting mold (12).
 2. The method as claimed in the preceding claim, wherein steps a) to c) are repeated continuously during the casting operations.
 3. The method as claimed in the preceding claim, wherein the flow characteristics are obtained by an analysis of the thermal characteristics of the steel in the casting mold (12).
 4. The method as claimed in the preceding claim, wherein the casting mold (12) has a pouring axis and is of the type consisting of an assembly of metal plates (22) backed by cooling devices (14) which are configured to allow the metal plates (22) to be cooled by the circulation of a cooling fluid, comprising an optical fiber (28), comprising a plurality of Bragg filters (34), extending in a wall of at least one of said plates (22), the optical fiber (28) extending in a direction not parallel to the pouring axis of the casting mold (12).
 5. The method as claimed in the preceding claim, further comprising the following steps: measurement of the temperature of at least one wall of the casting mold (12) by means of the optical fiber (28), and adjustment of the flow.
 6. (canceled)
 7. The method as claimed in claim 1, wherein the relative movement between the nozzle (11) and the casting mold (12) is effected in a direction parallel to the longitudinal axis of the casting mold (12).
 8. The method as claimed in claim 1, wherein the relative movement between the nozzle (11) and the casting mold (12) is effected by angularly offsetting the nozzle about the longitudinal axis of the casting mold (12).
 9. The method as claimed in claim 1, wherein the relative movement between the nozzle (11) and the casting mold (12) is effected both in a direction parallel to the longitudinal axis of the casting mold (12) and by angularly offsetting the nozzle about the longitudinal axis of the casting mold (12).
 10. The method as claimed in claim 1, wherein the nozzle (11) is secured to the tundish (8) and the relative movement between the nozzle (11) and the casting mold (12) is achieved by moving the tundish (8) with respect to the casting mold (12).
 11. A system for the continuous casting of liquid steel from a tundish to a continuous casting mold, comprising: a tundish (8), a casting mold (12) of the type consisting of an assembly of metal plates (22) backed by cooling devices (14) which are configured to allow the metal plates (22) to be cooled by the circulation of a cooling fluid, comprising an optical fiber (28), comprising a plurality of Bragg filters (34), extending in a wall of at least one of said plates (22), the casting mold having a pouring axis, the optical fiber (28) extending in a direction not parallel to the pouring axis of the casting mold (12), a protective nozzle (11) the lower end of which opens below the level of the steel in the casting mold (12) while the steel is being poured, the nozzle (11) being secured to the tundish (8), an emitter-receiver designed to send light into the optical fiber (28) and to receive the light reflected and/or transmitted by the optical fiber (28), a processor designed to: a) convert the data pertaining to the reflected and/or transmitted light received by the emitter-receiver into information pertaining to the flow in the casting mold (12), b) compare this information against a predefined model, c) determine the adjustment actions to be taken in order to balance the flow, d) emit a control signal, adjustment means (36) designed to receive the control signal and to adjust the flow of the steel in the casting mold (12) as a function of the control signal by effecting a relative movement between the nozzle and the casting mold.
 12. The system as claimed in the preceding claim, wherein the adjustment means (36) comprise a tundish car. 